A standard laboratory hood has a housing with an open or openable front side and with exhaust openings normally in the back wall which are connected to an exhaust fan for removing undesirable vapors, gases and the like from inside the hood. Thus it is possible for potentially dangerous experiments and tests to be carried out with relative safety.
A disadvantage of this system is that it frequently draws an inordinate amount of the ambient air in the laboratory out. Thus in seasons when the laboratory is heated or air conditioned a considerable waste of energy results. This tendency can be countered by reducing the volumetric throughput of the exhaust blower, with a concomitant decrease in hood efficiency.
It has been suggested to provide an outside-air intake for such a hood, to relieve the problem of robbing the laboratory of its climate-controlled or even filtered air. Such an arrangement has been found inadequate as the flow through the hood is uneven, so that dead corners result which are inadequately ventilated.